Blender also uses Z as up vector. I suspect 3D Studio is doing the same, and heaps has probably adopt the standard of its default tool.

We have grown accustomed to Z as depth with OpenGL and DirectX (and even before, i remember 3D tutorials on Amiga/ST were using Z as depth, probably because X,Y was «reserved» for 2D)
Even when you know that there are no standard for depth in coordinates systems, it’s a bit disturbing.